Commonly, three-dimensional ("3-D") seismic data acquisition involves arranging lines of seismic energy sources and receivers to form a rectangular grid. Each of the sources is activated to create a "seismic disturbance"--for example an explosive charge may be detonated. The disturbance creates a wave or ray that penetrates down into the layered strata of the earth. The wave contacts a sub-surface acoustic impedance boundary or layer and is reflected back to the surface to be detected by a patch of active receivers. Instrumentation, associated with the sources and receivers, converts reflected waves to signals which are recorded as data for post-acquisition processing and interpretation. The timing and amplitude of signals are instructive of sub-surface characteristics and features.
For a sub-surface layer with no dip and parallel with the seismic array, a wave reflects at a reflection point which lies directly beneath a geometric mid-point between a source and a receiver.
As stated, a plurality of sources and receivers are arranged in intersecting lines to form a grid and provide, in a first instance, a plurality of reflection points well distributed over the surface area of interest, and in a second instance, to provide multiple points of reflection which occur at a common mid-point ("CMP"). This technique of providing multiple reflections at a CMP is known as "stacking". Stacking results in an increase in the signal-to-noise ratio (S/N), improving the data. The number of multiple reflections at a CMP is known as the number of "fold".
Generally, a surface area to be surveyed is divided into small discrete rectangular areas known as "bins". The size of the bins is dependent upon the anticipated resolution required.
The prior art discloses a variety of seismic arrangements having a goal to obtain the greatest number of fold and the greatest number of CMPs, using the least number of sources and receivers.
Typically, the sources and receivers are equally spaced, along their respective lines, to lie at the mid-point of every second bin along that respective line. When the survey is performed, a common mid-point is produced at the center of each bin. U.S. Pat. No. 4,476,552 issued to Waters discloses a source and receiver arrangement that is typical of the arrangements commonly used. More particularly, Waters discloses:
--providing a plurality of equally-spaced seismic sources along a source line; PA0 --providing a plurality of equally-spaced seismic receivers along a receiver line, said receiver spacing being the same as the spacing for the source line; PA0 --placing a plurality of the source lines in an equally spaced and parallel fashion, the spacing or interval of the source lines being an even multiple of the receiver spacing; PA0 --placing a plurality of the receiver lines in an equally spaced and parallel fashion, the spacing of the receiver lines being an even multiple of the source spacing; PA0 --overlaying or "gridding" the source and receiver lines at right angles, so that a receiver and a source are identically positioned relative to each other (typically coincidentally aligned) at each crossing; ;and PA0 --producing multiple reflections at CMPs by creating a seismic disturbance at each source and detecting the disturbance at the receivers. PA0 --the receiver line spacing is a multiple of the source spacing (that is, the ratio of the receiver line spacing to the source spacing yields a remainder of zero); PA0 --the source line spacing is a multiple of the receiver spacing; and PA0 --the spacings are such so that, at every intersection of the source and receiver lines, the source and receiver are identically positioned relative to each other (typically coincident) at each crossing (see FIG. 5a). PA0 --the ratio between the receiver line spacing and the source spacing yields a remainder greater than zero; PA0 --the ratio between the source line spacing and the receiver spacing yields a remainder greater than zero; PA0 --there is an identically repeating occurrence of the relative offset between source and receiver at crossings of source and receiver lines in a grid, but only at every second, third, fourth . . . tenth source line and only at every second, third, fourth . . . tenth receiver line (see FIG. 7a); PA0 RLl=Sl(n.+-.1/S.sub.S) PA0 SLl=Rl(m.+-.1/S.sub.R) PA0 RLl=receiver line interval (or spacing), PA0 SLl=source line interval, PA0 Rl=receiver interval, PA0 Sl=source interval, PA0 S.sub.R =number of sub-bins in the receiver line direction, PA0 S.sub.S =number of sub-bins in the source line direction, and PA0 n, m=integers.
Noise is extraneous seismic interference which tends to mask actual sub-surface features identified by the reflected wave signals. Noise can make the data unusable. Typically, noise is compensated for by increasing the fold, thereby increasing the signal-to-noise ratio.
Unfortunately, compensation for noise is accompanied by several disadvantages.
With conventional seismic arrangements, an increase in fold can be accomplished by increasing the number of sources and receivers used in the grid. The associated increase in required equipment is expensive and requires anticipation of the expected significance of noise, before the survey is conducted. Further, there is no guarantee that the decisions made about the anticipated required fold are correct. Ultimately, an explorationist takes a risk and assumes a trade-off between economics and the quality of the data.
Should the noise be found to be worse than had been anticipated, a processing option is used to forestall discarding the data. The fold can be mathematically increased by combining adjacent, whole bins together. The bin size can be doubled in one dimension or quadrupled in two dimensions, thereby multiplying the data available in the new larger bin (higher fold).
The larger bin, and the associated increase in signal-to-noise ratio, is achieved only at the expense of a significantly poorer resolution (larger area), potentially obscuring sub-surface features of interest. The prior art has no processing options which would permit increasing the fold moderately to some intermediate value, perhaps just rendering the data usable, without the risk of going so far as to grossly diminish the resolution.
It is therefore an object of the present invention to provide a 3-D acquisition method which can reduce the risks for decisions made before the survey is conducted and further provide greater processing flexibility, to optimize the signal-to-noise ratio with resolution.